The present application is a U.S. non-provisional application based upon and claiming priority from Japanese Application No. 2001-159442, with a filing date of May 28, 2001, which is hereby incorporated by reference.
The present invention relates to polycarbonates for optical use, and in particular to polycarbonates for optical use which have better color stability and formability, making them especially suitable for optical disk substrates.
Polycarbonates have exceptional mechanical properties such as impact resistance, as well as other properties such as heat resistance and transparency, and are thus widely used in applications such as various mechanical parts, optical disks, and automobile parts. They are particularly promising for optical applications such as optical disks for memory, fiber optics, and lenses, and are the subject of considerable research.
Methods known for the production of such polycarbonates include directly bringing about a reaction between a phosgene and a bisphenol such as bisphenol A (interfacial method), or bringing about the melt polycondensation of a bisphenol such as bisphenol A and a carbonic diester such as diphenyl carbonate (transesterification).
A drawback of the interfacial method, which features the use of phosgenes and also employs large amounts of solvent such as methylene chloride, is the extreme difficulty involved in getting rid of the chlorine, which makes this not an altogether desirable option when producing polycarbonates for optical use.
An advantage of melt polycondensation, on the other hand, is that polycarbonates can be produced at a lower cost compared to the interfacial method. In addition, no toxic substances such as phosgenes are used, and there is no need for solvents such as methylene chloride, making this an extremely attractive way to produce polycarbonates for optical use.
However, the molecular weight must be adjusted to a low level within a narrow range in order to ensure that polycarbonates used as optical materials will result in a disk having a certain level of strength and to allow the transfer of fine pits and grooves on the surface during injection molding.
However, as noted in Japanese Unexamined Patent Publication (Kokai) 11-300842, even with the use of such low molecular weight polycarbonates, the material must be formed under extremely limited conditions in which the difference between the glass transition temperature of the polycarbonate resin and the temperature to which the mold is set on the stamper side is between 0 and 7xc2x0 C., which can be a drawback in terms of mass production.
The addition of 3.5 to 8 wt % of low molecular weight material represented by the following formula for polycarbonates for optical use has been proposed in Japanese Unexamined Patent Publication (Kokai) 11-35671 in an effort to improve the transfer properties during injection molding, and the copolymerization of a polysiloxane compound or the like with polycarbonates has also been proposed, as disclosed in Japanese Unexamined Patent Publication (Kokai) 10-158499. 
(where X is a C1 to C4 alkylene, C2 to C3 alkylidene, oxygen atom, sulfur atom, carbonyl, sulfinyl, or sulfone, and n is an integer of 1 to 4).
However, problems encountered when blending such low molecular weight polycarbonates are the considerable production of gas during molding, and the extensive mold staining during continuous production, resulting in unsatisfactory productivity. In addition, products obtained through the copolymerization of polysiloxane compounds require a considerable investment in the manufacture of the polycarbonates, while equipment which is different from conventional optical disk manufacturing equipment is also required due to differences in the optical properties and formability of the copolymer polycarbonates that are obtained.
As a result of extensive research in view of the foregoing, the inventors perfected the present invention upon discovering that better formability and color stability as well as better transfer properties could be obtained during the injection molding of optical disks by using polycarbonates obtained through melt polycondensation, where such polycarboxylates for optical use comprise a bisphenol and carbonic diester which have undergone melt polycondensation in the presence of an alkaline compound catalyst, said polycarbonates for optical use wherein in that:
(i) the intrinsic viscosity (IV) as determined at 20xc2x0 C. in methylene chloride is between 0.34 and 0.38;
(ii) the glass transition temperature (Tg) as determined by DSC is between 143 and 147xc2x0 C.; and
(iii) the intrinsic viscosity (IV) and glass transition temperature (Tg) comply with the following formula (A):
IVxc3x9795.888+107.9 less than Tg less than IVxc3x9795.888+113.7xe2x80x83xe2x80x83(A) 
In view of the conventional technology described above, an object of the present invention is to provide polycarbonates for optical use with better formability and color stability at elevated temperatures, as well as applications thereof.
The polycarbonates for optical use (resin compositions) of the present invention are polycarbonates obtained upon melt polycondensation of a bisphenol and a carbonic diester in the presence of an alkaline compound catalyst, where the polycarbonates have the properties defined in (i) through (iii) above.
The aforementioned bisphenols should substantially comprise bisphenol A.
The polycarbonates for optical use should comprise no more than 1.5 wt % fractions with a molecular weight of 1,000 or less, as determined by GPC.
Optical disk substrates of the present invention are formed of the aforementioned polycarbonates for optical use.